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Serviceman's Log

Faulty electrolytic capacitors have been a common problem in electronics gear over the last few years. In fact, some items only just make it through warranty before their power supply electros develop lumpy bits and fail.

by the Serviceman

Items Covered This Month

• DGTEC DG-HD03 set-top box

• Hot over coolant

• The bouncing modem

• Front-panel USB socket woes

Marilyn asked me to look at her DGTEC DG-HD03 set-top box (STB). The unit was barely 18 months old but the display was now stubbornly showing “8888” and nothing else. It wouldn’t respond to commands from the remote and there was no output signal.

Click for larger image

It hadn’t been a sudden failure. Instead, the unit had become increasingly intermittent in operation over a period of several weeks before finally packing it in. Marilyn had tried to clear the fault by switching it off and on at the wall socket to reboot the firmware but it made no difference.

The unit was located in a TV cabinet in the living room. It rested on top of the DVD player which in turn sat on a shelf below the TV set. And one thing I noticed as it was being retrieved was that there wasn’t a lot of space between the STB and the shelf above it.

That meant that the ventilation wasn’t all that great but at the time, I mentally dismissed this as being a factor. These devices don’t consume much power, especially on standby, so overheating shouldn’t have been a problem (or so I thought).

My initial reaction was that the fault might be due to “crook” electrolytic capacitors, probably in the power supply. I’ve lost count of the number of devices I’ve repaired over the years simply by replacing faulty electros. They’re usually quite easy to spot too – they’re the ones with the bulging (domed) tops.

Click for larger image

I told Marilyn that if it was something obvious such as faulty electros, then fixing it shouldn’t be a problem. But if it wasn’t obvious, then it would be a “bin job” as the fault would be virtually impossible to track down, especially on a double-sided PC board with lots of surface-mount parts (and no circuit diagram).

When it came time to tackle the unit, I confirmed the symptoms and checked that the batteries in the remote were OK. I then removed the case lid and took a peek inside. This revealed two PC boards – a power supply board and a much larger main board which carried the microcontroller, digital tuner, support circuitry and the various input and output sockets. And the fault was glaringly obvious – five electrolytic capacitors with bulging tops on the power supply board!

The other thing that struck me was that the varnish on the top of the power supply board had discoloured around many of the components. So it looked like it had been running hot after all and this had undoubtedly contributed to the premature failure of the electros.

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